1. Field of the Invention
This invention relates to a process for carrying out an endothermic or exothermic reaction using a moving bed of magnetically stabilized particles wherein the reaction temperature profile is controlled to an optimum level for maximum product yield, selectively and/or conversion. This invention is particularly suited to reaction processes constrained by equilibrium limitations, such as synthesis of ammonia or methanol, and to reaction processes where it is important to limit maximum reactor temperature to avoid producing undesirable by-products, such as epoxidation of ethylene.
2. Description of Related Disclosures
Conversion of hydrogen and nitrogen to ammonia generally occurs in the presence of a promoted iron catalyst at temperatures between about 343.degree. C. and 538.degree. C. As the reaction is exothermic, releasing about 23,000 Btu per pound-mole of ammonia produced, the heat generated in the catalyst bed must be removed by some means to control bed temperatures and provide an optimum temperature profile for efficient catalyst utilization.
In designing reactors for the synthesis of ammonia, several factors must be considered. High pressure is required to shift the reaction equilibrium significantly toward conversion to ammonia. Also, lower temperatures favor higher equilibrium concentrations of ammonia at a given pressure, but excessively low temperatures are associated with slow kinetic rates of reaction. Therefore, the optimal temperatures profile associated with the reaction which maximizes conversion to ammonia for a given inlet gas pressure, temperature and composition is a compromise between adequate equilibrium concentration of ammonia and a suitably rapid rate of reaction.
Many of the present commercial reactors for synthesis of ammonia from hydrogen and nitrogen constitute fixed bed reactors, which suffer from at least two limitations. First, large catalyst particles, which reduce surface area per unit reactor volume and which retard the reaction rate, must be employed to prevent the pressure drop across the bed from being prohibitively large. Secondly, the optimal temperature profile which maximizes conversion in the reactor is difficult to achieve. Conventional reactor designs attempting to approximate the optimal temperature profile include adiabatic beds in series with quenches between the beds, adiabatic beds in series with heat exchangers between the beds, and use of catalyst beds with cooling tubes installed within the bed. However, the effectiveness of interstage quenching and cooling (as takes place in a multiple adiabatic bed design) is limited by the significant temperature rise which occurs, resulting in the operation of a major portion of the catalytic volume at a temperature far from the optimum reaction temperature for conversion. While the use of cooling tubes in the bed itself can, theoretically, provide a closer approach to the optimum reaction temperature profile, the cooling surface to catalyst volume ratio is fixed by the design and it is not possible to make adjustments for different operating conditions and catalyst activity so as to maintain an optimum reaction temperature profile throughout the life of the catalyst charge.
The use of conventional stationary fluidized beds without magnetic stabilization for ammonia synthesis allows for smaller particles to be used while avoiding any large pressure drops but does not permit an optimal temperature profile to be attained for maximum conversion.
In other exothermic reactions such as partial oxidation of hydrocarbons, optimum temperature profiles are important when unacceptable side reactions such as combustion occur at high temperatures so as to affect adversely the selectivity for the desired product. Thus, for example, it has been found that the conversion of ethylene to ethylene oxide using a fixed silver catalyst bed is limited to about 20-25% to obtain acceptable selectivity (at least about 70%) to ethylene oxide. This necessitates substantial recycling of the unconverted ethylene. Attempts to achieve higher conversions result in releasing more heat and lowering the selectivity because higher temperatures favor complete oxidation of ethylene to carbon dioxide and water. Present technology utilizes complicated cooling systems such as shell and tube type heat exchangers to control the reaction temperature of the fixed bed reactor. However, selectivity is limited to about 70% and conversion per pass is limited to 20-25% of the ethylene feed. Recycling the unreacted ethylene requires cool-down and recompression, and "hot spots" due to fluid "dead spaces" in the cooling medium and shell-and-tube type heat exchangers are often problematical with respect to temperature runaway. Heat transfer also presents a problem in the synthesis of hydrocarbons from carbon monoxide and hydrogen.
Numerous workers have studied the influence of magnetization on the dynamics of fluidized solids in batch beds. An early account of this phenomenon was reported by M. V. Filippov, Applied Magnetohydrodynamics, Trudy Instituta Fizika Akad. Nauk., Latviiskoi SSR 12, 215 (1960). Subsequent workers have observed the influence which magnetization exerts on pulsations, heat transfer, structure and other characteristics of magnetized and fluidized solids in batch beds. A review of some of this work is provided by Bologa and Syutkin, Elektron Obrab Mater, 1, 37 (1977). In particular, a report of the effect of electromagnetic fields on the heat transfer process between the heating surface and the fluidized bed is given by S. V. Syutkin et al., Elektron Obrab Mater, 6, 61 (1976). In addition, studies of the effect of macrokinetics on the value of optimal temperatures were made in a fluidal layer of catalyst by the flow method in the synthesis of ammonia and methanol by I. A. Zrenchev, Dokl. Bolg. Akad. Nauk, 27, 1501 (1974). Invanov and coworkers have described some benefits of using an applied magnetic field on a stationary bed of fluidized ferromagnetic solids in the synthesis of ammonia and some of the characteristics of the bed for this process. See U.K. Pat. No. 1,148,513 and numerous publications by the same authors, e.g., Ivanov et al., Intern. Chem. Eng., 15, 557 (1975), Ivanov et al., Dokl. Bolg. Akad. Nauk, 28, 55 (1975) and Ivanov et al., Dolk. Bolg. Akad. Nauk, 22, 1405 (1969), which report that the use of the magnetic field restricts carryover of catalyst particles to the effluent gas stream. In addition, I. Zrenchev et al., Khim. I. Industriya, 51, 256 (1979) relates to ammonia synthesis in a fixed bed in the presence of a magnetic field. The use of magnetically stabilized fluidized beds in general and the use thereof in a plurality of chemical reactions, separations and other applications are disclosed in U.S. Pat. No. 4,115,927. Additionally, a method for controlling temperatures of exothermic reactions using a magnetic field is described in U.S. Pat. No. 2,519,481.
U.S. Pat. Nos. 4,247,987 and 4,283,204 describe the use of a magnetically stabilized bed to facilitate circulating solids countercurrently to the fluidizing medium whereby the solids and gases flow countercurrently to each other in plug-flow fashion. The disclosed applications of these concepts are in the recovery of solids from the main reaction or adsorption zone, the transfer of the solids to a regeneration zone and the clean-up of the solids prior to return to the main reaction or adsorption zone.